This objective of this grant is to design a fourth and final vision of the Dual-Purkinje-Image eyetracker and to make this instrumentation available to the eyetracker-user community. The following optical and electronic changes in progress will represent major improvements over the third generation instrument: automatic alignment to the subject/patient; improved noise level to below one minute of arc; improved thermal stability; increased frequency response (total system delay on the order of 0.5 ms): improved blink and recovery circuits; and built-in calibration. Studies are also underway to maximize the dimensions of the tracking field.